Contactless acceleration switch

ABSTRACT

A contactless acceleration switch detects a threshold acceleration value when a mass attached to a spring, moves towards a source, a drain, and a threshold adjustment channel implanted in a substrate layer. The threshold adjustment channel is located between the source and the drain. The implanted area is located between insulator posts. A spring is attached to the insulator posts. A mass is held above the implanted area by the spring. When the threshold acceleration value is detected, the mass moves towards the substrate layer. The threshold adjustment channel then inverts causing current to flow between the source and the drain, providing an electrical signal indicating that the threshold acceleration value has been reached.

FIELD

[0001] The present invention relates generally to acceleration switches,and more particularly, relates to a semiconductor acceleration switchwithout metal contacts.

BACKGROUND

[0002] Acceleration switches are designed to switch on or off when athreshold acceleration value of acceleration is detected. An example ofan application using acceleration switches is air bag systems. Theacceleration switch in an airbag system will detect a suddende-acceleration of the vehicle. When the threshold acceleration value isreached, the contacts on the switch close, sending a signal to thecontrol module. Acceleration switches are also used in free falldetection systems for elevators, seat belt sensors, and machinemonitoring of excess vibrations.

[0003] Acceleration switches were initially mechanical switches thatincluded a spring loaded mass. Today, acceleration switches are morecommonly manufactured using micromachining. Micromachining typicallyinvolves combining electronics and tiny mechanical components on asemiconductor chip. Initial designs used bulk micromachining, whichconsists of making micromechanical devices by etching into the siliconwafer. Bulk micromachining makes extensive use of wafer bonding, whichis the process of permanently joining different silicon wafers together.

[0004] Further improvements in the manufacture of acceleration switcheshave been made using surface micromachining. Surface micromachiningfabricates micromechanical devices on the surface of a silicon wafer.The features of the device are built up layer by layer through acombination of deposition, patterning, and etching stages. Thistechnique is compatible with other semiconductor processing that may beperformed on the same wafer for other purposes. Results from surfacemicromachining may be more uniform and more repeatable than thoseobtained from bulk micromachining.

[0005] Typical acceleration switches have contained at least oneelectrical contact. The contacts are designed to close when thethreshold acceleration value is detected. The contacts are formed with ametal, such as gold. However, there may be problems with the use ofmetal contacts, such as microwelding, arcing, and oxidation. Theseproblems may contribute to the failure of the switch.

[0006] It would be desirable to have an acceleration switch that doesnot employ a metal contact to signal when the threshold accelerationvalue is detected.

SUMMARY

[0007] In accordance with this invention, a contactless accelerationswitch contains a substrate layer containing a source, a drain, athreshold adjustment channel, at least two insulator posts, a mass, aspring, and a gate insulating layer. The source, the drain, thethreshold adjustment channel, and the gate insulating layer are locatedbetween the at least two insulator posts. The spring is attached to eachof the at least two insulator posts and supports the mass, above thesubstrate layer.

[0008] The contactless acceleration switch is made as follows. Implantthe source, the drain, and the threshold adjustment channel in thesubstrate layer such that the threshold adjustment channel is locatedbetween the source and the drain. Form the at least two insulator postson the substrate layer such that the source, the drain, and thethreshold adjustment channel are located between the at least twoinsulator posts. Form a first sacrificial layer on the substrate layerbetween the at least two insulator posts. Form the mass on the firstsacrificial layer. Form a second sacrificial layer shaped to provide apattern for forming the spring. Form the spring. Remove the firstsacrificial layer and the second sacrificial layer. Form the gateinsulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Preferred embodiments are described below in conjunction with theappended drawing figures, wherein like reference numerals refer to likeelements in the various figures, and wherein:

[0010]FIG. 1 is a cross-sectional view of a contactless accelerationswitch according to an exemplary embodiment;

[0011] FIGS. 2-5 are cross-sectional views of the contactlessacceleration switch of FIG. 1 at various stages of fabrication accordingto an exemplary embodiment; and

[0012]FIG. 6 is a flow chart of the fabrication process of a contactlessacceleration switch according to an exemplary embodiment.

DETAILED DESCRIPTION

[0013]FIG. 1 is a cross sectional view of a contactless accelerationswitch 100 according to an exemplary embodiment. The figures are notdrawn to scale and are approximations of an exemplary embodiment. Forexample, corners may be rounded in an exemplary embodiment, rather thanstraight as depicted. Contactless acceleration switch 100 includes asubstrate layer 102, a source 104, a drain 106, a threshold adjustmentchannel 108, at least two insulator posts 110, a mass 112, a spring 114,and a gate insulating layer 120.

[0014] The substrate layer 102 is the underlying material upon which thecontactless acceleration switch 100 is fabricated. The substrate layer102 may be composed of a semiconductor material such as silicon orgallium arsenide. Silicon is the preferred material for the substratelayer 102 in an exemplary embodiment. The substrate layer 102 may bedoped with either a P-type or N-type dopant.

[0015] The source 104 and the drain 106 may be substantially located inthe substrate layer 102. The source 104 and the drain 106 may be dopedbased on whether the substrate layer 102 has been doped. Typically thesource 104 and the drain 106 are doped with the opposite dopant type asthe substrate layer 102. For example, if the substrate layer 102 hasbeen doped with a P-type dopant, the source 104 and the drain 106 may bedoped with an N-type dopant. Ion implantation may be employed to dopethe source 104 and the drain 106; however, other methods may beemployed.

[0016] The threshold adjustment channel 108 may be located between thesource 104 and the drain 106, substantially within the substrate layer102. The threshold adjustment channel 108 may be doped to a level thatmay cause the threshold adjustment channel 108 to invert when the mass112 moves substantially towards the substrate layer 102. Ionimplantation may be employed to dope the threshold adjustment channel108; however, other methods may be suitable for this purpose. Thethreshold adjustment channel 108 may be doped with either P-type orN-type dopant. In an exemplary embodiment, the threshold adjustmentchannel 108 may be doped with the opposite dopant type as the source 104and the drain 106. However, some embodiments will use the same dopanttype for the threshold adjustment channel 108, the source 104, and thedrain 106.

[0017] The gate insulating layer 120 may be located substantially abovethe source 104, the drain 106, and the threshold adjustment channel 108.The gate insulating layer 120 may substantially limit electricconduction between the mass 112 and the substrate layer 102 when themass 112 moves substantially towards the substrate layer 102. The gateinsulating layer 120 may be composed of an insulating material, such assilicon dioxide or silicon nitride. Silicon dioxide is the preferredmaterial for the gate insulating layer 120 in an exemplary embodiment.

[0018] The at least two insulator posts 110 may be composed of aninsulating material, such as silicon dioxide or silicon nitride. In anexemplary embodiment, the at least two insulator posts 110 are composedof silicon dioxide. The at least two insulator posts may besubstantially located on the substrate layer 102. The source 104, thedrain 106, the threshold adjustment channel 108, and the gate insulatinglayer 120 may be substantially located between the at least twoinsulator posts 110. The at least two insulator posts may provide aninterface between the substrate layer 102 and the spring 114.

[0019] The mass 112 may be formed with an electrically conductivematerial, such as metal or doped silicon. In an exemplary embodiment,the mass 112 is composed of doped silicon. The mass 112 may operate as amoveable gate in combination with the source 104, and the drain 106,forming a field effect transistor (FET). The mass 112 may be locatedsubstantially above the source 104, the drain 106, the thresholdadjustment channel 108, and the gate insulating layer 120.

[0020] A substantially constant voltage may be applied between the mass112 and the substrate layer 102. The voltage may be determined usingfactors such as mass size, spring constant, operation range, andhysteresis. The voltage may range from less than five volts for lowacceleration range devices to hundreds of volts for large accelerationrange devices. The voltage amount may also provide a method of adjustinga threshold acceleration value. The threshold acceleration value may bethe value of acceleration at which the contactless acceleration switch100 may be triggered.

[0021] An air gap may be located between the mass 112 and the gateinsulating layer 120 when an acceleration level is below the thresholdacceleration value. The thickness of the air gap may be dependant uponthe value of the substantially constant voltage applied between the mass112 and the substrate 102, and the threshold acceleration value. The airgap thickness for a low range value of acceleration that may trigger thecontactless acceleration switch 100 may be greater than 1000 Angstroms.

[0022] The spring 114 may be formed with an electrically conductivematerial, such as metal or doped silicon. In an exemplary embodiment,the spring 114 is composed of doped silicon. The spring 114 may beattached to each of the at least two insulator posts 110. The mass 112may be attached to substantially the center of the spring 114.

[0023] Below the threshold acceleration value, the mass 112 may not beattracted to the substrate layer 102 and the threshold adjustmentchannel 108 may not be inverted. When the acceleration level exceeds thethreshold acceleration value, an electric field between the mass 112 andthe substrate layer 102 may form. The electric field may create anelectrostatic force, which may attract the mass 112 to the substratelayer 102. The mass 112 may move towards the substrate layer 102 to aposition of critical distance. The position of critical distance may bethe distance between the mass 112 and the substrate layer 102 at whichpoint the electrostatic force exceeds a spring force. The spring forcemay be a force of the spring 114 that operates to maintain the air gapbetween the mass 112 and the substrate layer 102.

[0024] Once the mass 112 reaches the position of critical distance, themass 112 may suddenly contact the gate insulating layer 120, eliminatingthe air gap. In other words, some or all of the mass 112 may contact atleast a portion of the gate insulating layer 120. When the mass 112substantially contacts the gate insulating layer 120, the strength ofthe electric field may be at a maximum level. The threshold adjustmentchannel 108 may invert, allowing current to flow between the source 104and the drain 106, which may turn on the FET. The threshold accelerationvalue may be determined by the initial air gap thickness, the positionof critical distance, the spring strength, and the value of thesubstantially constant voltage applied between the mass 112 and thesubstrate layer 102.

[0025] The source 104 and the drain 106 may act as electrodes, providingan electrical signal indicating that the threshold acceleration valuehas been reached. The electrical signal may be sent to a controller forfurther processing. The electrical signal may be a value of voltage,current, or resistance. As described, the contactless accelerationswitch 100 may substantially provide on-off switching without the needfor metal contacts.

[0026]FIG. 2 is a cross sectional view of the contactless accelerationswitch 100 during the initial steps of processing according to anexemplary embodiment. The contactless acceleration switch 100 may befabricated using surface micromachining technology. Manufacturing of thecontactless acceleration switch 100 may begin by performing the source104, drain 106, and threshold adjustment channel 108 implants. Ionimplantation may be used to dope the source 104, the drain 106, and thethreshold adjustment channel 108 regions in the substrate layer 102.Other doping methods that are compatible with the substrate layer 102may also be used. As previously discussed, the substrate layer 102, thesource 104, the drain 106, and the threshold adjustment channel 108 maybe doped with either P-type or N-type dopant. In an exemplaryembodiment, the substrate layer 102 and the threshold adjustment channel108 are doped with the opposite dopant type as the source 104 and thedrain 106. However, other embodiments may use a different selection ofdopant types.

[0027]FIG. 3 is a cross sectional view of the contactless accelerationswitch 100 during additional stages of processing according to anexemplary embodiment. The at least two insulator posts 110 areinstalled. An insulating material, such as silicon dioxide or siliconnitride, may be deposited on the substrate layer 102. In an exemplaryembodiment silicon dioxide is used. The at least two insulator posts 110are thermally grown in an exemplary embodiment. However, the at leasttwo insulator posts 110 may also be deposited using low temperatureoxidation, low pressure chemical vapor deposition, sputtering, or otherdeposition techniques. A photoresist may be placed on the insulatinglayer to define the location of the at least two insulator posts 110. Inan exemplary embodiment, the at least two insulator posts 110 are thenformed using a wet etch technique. Other etching techniques, such asplasma etching, may also be suitable. The photoresist may then beremoved.

[0028]FIG. 4 is a cross sectional view of the contactless accelerationswitch 100 during additional stages of processing according to anexemplary embodiment. After the at least two insulator posts 110 areformed, a first sacrificial layer 116 may be deposited. The firstsacrificial layer 116 may be composed of silicon dioxide, polymide,photoresist, various polymers, doped silicon, or metals. The selectionof the first sacrificial layer material may depend upon the materialselected for the mass 112 and the spring 114, and which etchingtechnique may be employed. In an exemplary embodiment, the firstsacrificial layer 116 is composed of silicon dioxide.

[0029] The first sacrificial layer 116 may be deposited substantiallybetween the at least two insulator posts 110. The deposition methodselected may be chosen based on the material used for the firstsacrificial layer 116. In an exemplary embodiment, the first sacrificiallayer 116 is thermally grown. The first sacrificial layer 116 may alsobe deposited using low temperature oxidation, low pressure chemicalvapor deposition, sputtering, or other deposition techniques. The firstsacrificial layer 116 is then patterned and etched to form asubstantially continuous layer between the at least two insulator posts110. A wet etch is preferred in an exemplary embodiment; however, otheretching techniques, such as plasma etching, may also be suitable forthis purpose.

[0030] The mass 112, which operates as a moveable gate, may then bedeposited on the first sacrificial layer 116 using a low pressurechemical vapor deposition process. Other deposition methods, such assputtering, may also be used. The mass 112 is then patterned and etched.A plasma etch is employed in an exemplary embodiment; however, otheretching methods may also be suitable for this purpose.

[0031]FIG. 5 is a cross sectional view of the contactless accelerationswitch 100 during additional stages of processing according to anexemplary embodiment. A second sacrificial layer 118 may then bedeposited. The second sacrificial layer 118 may be composed of silicondioxide, polymide, photoresist, various polymers, doped silicon, ormetals. The selection of the second sacrificial layer material maydepend upon the material selected for the mass 112 and the spring 114,and which etching technique may be employed. In an exemplary embodiment,the second sacrificial layer 118 is composed of silicon dioxide.

[0032] The deposition method employed may be chosen based on thematerial selected for the second sacrificial layer 118. In an exemplaryembodiment, low temperature oxidation is employed. However, otherdeposition techniques, such as tetraetheyl orthosilicate, low pressurechemical vapor deposition, sputtering, or other deposition techniques,may also be suitable for this purpose. The second sacrificial layer 118is then patterned to provide interconnection between the mass 112 andthe at least two insulator posts 110. An etching process is thenemployed to form the second sacrificial layer 118. In an exemplaryembodiment a wet etch is used; however, other etching techniques, suchas plasma etching, may be employed. The shape of the second sacrificiallayer may be designed to provide a pattern for forming the spring 114.

[0033] The spring material is then deposited substantially on the secondsacrificial layer 118 and the at least two insulator posts 110. Thedeposition process employed may be chosen to be compatible with thespring material chosen. Low pressure chemical vapor deposition may beused in an exemplary embodiment; however, other deposition techniques,such as sputtering, may also be suitable for this purpose. The springmaterial may then be patterned and etched to substantially form thespring 114. An exemplary embodiment employs a plasma etch. Other etchingtechniques, such as a wet etch, may also be suitable for this purpose.

[0034] The first sacrificial layer 116 and the second sacrificial layer118 may then be substantially removed, leaving the spring 114 and themass 112 as a free standing structure (see FIG. 1). The firstsacrificial layer 116 and the second sacrificial layer 118 may beremoved by etching. Any etching process that is selective to thematerial of the sacrificial layers may be used. A wet etch may be usedin an exemplary embodiment to remove the first sacrificial layer 116 andthe second sacrificial layer 118. An air gap may be locatedsubstantially between the mass 112 and the substrate layer 102 after thefirst sacrificial layer 116 is removed.

[0035] The gate insulating layer 120 may be deposited on the substratelayer 102, substantially above the source 104, the drain 106, and thethreshold adjustment channel 108. In a preferred embodiment, silicondioxide may be thermally grown on the substrate layer 102 between thetwo insulator posts 110 after the first sacrificial layer 116 isremoved. In an alternative embodiment, the gate insulating layer 120 maybe deposited before the deposition of the first sacrificial layer 116.When the first sacrificial layer 116 is removed, the gate insulatinglayer 120 may be uncovered. This alternative embodiment may be employedwhen etching the first sacrificial layer 116 will not damage theunderlying gate insulating layer 120. For example, if the firstsacrificial layer materials are metal, an etch may be chosen that mayremove the first sacrificial layer 116 without damaging the gateinsulating layer 120.

[0036]FIG. 6 provides a flow chart of fabrication process 600 of thecontactless acceleration switch 100 according to an exemplaryembodiment. Fabrication process 600 summarizes the process describedabove with reference to FIG. 2 through FIG. 5. This process may becompatible with complementary metal oxide semiconductor (CMOS)fabrication.

[0037] The contactless acceleration switch 100 may also be employed in amanner to detect when the acceleration level goes below the thresholdacceleration value. The mass 112 may move away from the substrate layer102 when the acceleration level goes below the threshold accelerationvalue. The air gap may be formed between the mass 112 and the substratelayer 102. The electrical signal generated may indicate that theacceleration level is below the threshold acceleration value.

[0038] It should be understood that the illustrated embodiments areexemplary only, and should not be taken as limiting the scope of thepresent invention. For example, a variety of semiconductor fabricationtechniques, including various methods of deposition and etching, may beemployed without departing from the scope of the invention itself. Theclaims should not be read as limited to the described order or elementsunless stated to that effect. Therefore, all embodiments that comewithin the scope and spirit of the following claims and equivalentsthereto are claimed as the invention.

We claim:
 1. A contactless acceleration switch system, comprising incombination: a substrate layer containing a source, a drain, and athreshold adjustment channel; a gate insulating layer locatedsubstantially above the source, the drain, and the threshold adjustmentchannel; at least two insulator posts, wherein the source, the drain,the threshold adjustment channel, and the gate insulating layer arelocated substantially between the at least two insulator posts; a mass;and a spring substantially supporting the mass above the substratelayer, wherein the spring is attached to each of the at least twoinsulator posts.
 2. The system of claim 1, wherein the substrate layeris composed of a semiconductor material.
 3. The system of claim 2,wherein the semiconductor material is silicon.
 4. The system of claim 1,wherein the gate insulating layer is composed of silicon dioxide.
 5. Thesystem of claim 1, wherein the at least two insulator posts are composedof an insulating material.
 6. The system of claim 5, wherein theinsulating material is silicon dioxide.
 7. The system of claim 1,wherein the mass is composed of an electrically conductive material. 8.The system of claim 7, wherein the electrically conductive material isdoped silicon.
 9. The system of claim 1, wherein the spring is composedof an electrically conductive material.
 10. The system of claim 9,wherein the electrically conductive material is doped silicon.
 11. Thesystem of claim 1, wherein the threshold adjustment channel is doped toa level to cause the threshold adjustment channel to invert when themass moves substantially towards the substrate layer.
 12. The system ofclaim 1, wherein the gate insulating layer substantially limits electricconduction between the mass and the substrate layer.
 13. The system ofclaim 1, wherein the mass operates as a moveable gate.
 14. The system ofclaim 1, wherein the mass, the source, and the drain operate as a fieldeffect transistor.
 15. The system of claim 1, wherein an air gap islocated substantially between the mass and the substrate layer when anacceleration level is substantially below a threshold accelerationvalue.
 16. The system of claim 1, wherein the mass moves substantiallytowards the substrate layer when a threshold acceleration value isdetected.
 17. The system of claim 16, wherein the threshold adjustmentchannel inverts when the mass moves towards the substrate layer.
 18. Thesystem of claim 17, wherein current flows between the source and thedrain when the threshold adjustment channel inverts.
 19. The system ofclaim 1, wherein the source and the drain act as electrodes providing anelectrical signal that indicates that a threshold acceleration value isdetected.
 20. The system of claim 1, wherein a substantially constantvoltage is applied between the mass and the substrate layer.
 21. Thesystem of claim 20, wherein the substantially constant voltage isdetermined by factors selected from the group consisting of mass size,spring constant, operation range, and hysteresis.
 22. A contactlessacceleration switch system, comprising in combination: a siliconsubstrate layer containing a source, a drain, and a threshold adjustmentchannel, wherein the threshold adjustment channel is doped to a level tocause the threshold adjustment channel to invert when a mass movessubstantially towards the silicon substrate layer, and wherein thesource and the drain act as electrodes providing an electrical signalthat indicates that a threshold acceleration value is detected; a gateinsulating layer located substantially above the source, the drain, andthe threshold adjustment channel, wherein the gate insulating layer iscomposed of silicon dioxide, and wherein the gate insulating layersubstantially limits electric conduction between the mass and thesilicon substrate layer; at least two insulator posts composed ofsilicon dioxide, wherein the source, the drain, the threshold adjustmentchannel, and the gate insulating layer are located substantially betweenthe at least two insulator posts; the mass composed of doped silicon,wherein the mass operates as a moveable gate, wherein the mass, thesource, and the drain operate as a field effect transistor, wherein anair gap is located substantially between the mass and the siliconsubstrate layer when an acceleration level is substantially below thethreshold acceleration value, wherein the mass moves substantiallytowards the silicon substrate layer when the threshold accelerationvalue is detected, wherein the threshold adjustment channel inverts whenthe mass moves towards the silicon substrate layer, wherein currentflows between the source and the drain when the threshold adjustmentchannel inverts, and wherein a substantially constant voltage is appliedbetween the mass and the silicon substrate layer; and a spring composedof doped silicon substantially supporting the mass above the siliconsubstrate layer, wherein the spring is attached to each of the at leasttwo insulator posts.
 23. A method for making a contactless accelerationswitch, comprising in combination: implanting a source, a drain, and athreshold adjustment channel in a substrate layer, and wherein thethreshold adjustment channel is located substantially between the sourceand the drain; forming at least two insulator posts on the substratelayer, and wherein the source, the drain, and the threshold adjustmentchannel are located substantially between the at least two insulatorposts; forming a first sacrificial layer on the substrate layersubstantially between the at least two insulator posts; forming a masson the first sacrificial layer; forming a second sacrificial layershaped to provide a pattern for forming a spring; forming the spring;removing the first sacrificial layer and the second sacrificial layer,and wherein the spring holds the mass substantially above the substratelayer; and forming a gate insulating layer.
 24. The method of claim 23,wherein the substrate layer is composed of a semiconductor material. 25.The method of claim 24, wherein the semiconductor material is silicon.26. The method of claim 23, wherein ion implantation is used to implantthe source, the drain, and the threshold adjustment channel in thesubstrate layer.
 27. The method of claim 23, wherein the thresholdadjustment channel is doped to a level to cause the threshold adjustmentchannel to invert when the mass moves substantially towards thesubstrate layer.
 28. The method of claim 23, wherein the at least twoinsulator posts are composed of an insulating material.
 29. The methodof claim 28, wherein the insulating material is silicon dioxide.
 30. Themethod of claim 23, wherein the at least two insulator posts arethermally grown.
 31. The method of claim 23, wherein wet etching is usedto form the at least two insulator posts.
 32. The method of claim 23,wherein the first sacrificial layer is composed of a material selectedfrom the group consisting of silicon dioxide, polymide, photoresist,polymer, doped silicon, and metal.
 33. The method of claim 23, whereinthe first sacrificial layer is composed of silicon dioxide.
 34. Themethod of claim 23, wherein the first sacrificial layer is thermallygrown.
 35. The method of claim 23, wherein wet etching is used to formthe first sacrificial layer.
 36. The method of claim 23, wherein themass is composed of an electrically conductive material.
 37. The methodof claim 36, wherein the electrically conductive material is dopedsilicon.
 38. The method of claim 23, wherein the mass is deposited usinglow pressure chemical vapor deposition.
 39. The method of claim 23,wherein plasma etching is used to form the mass.
 40. The method of claim23, wherein the second sacrificial layer is composed of a materialselected from the group consisting of silicon dioxide, polymide,photoresist, polymer, doped silicon, and metal.
 41. The method of claim23, wherein the second sacrificial layer is composed of silicon dioxide.42. The method of claim 23, wherein low temperature oxidation is used todeposit the second sacrificial layer.
 43. The method of claim 23,wherein wet etching is used to form the second sacrificial layer. 44.The method of claim 23, wherein the spring is composed of anelectrically conductive material.
 45. The method of claim 44, whereinthe electrically conductive material is doped silicon.
 46. The method ofclaim 23, wherein low pressure chemical vapor deposition is used todeposit the spring.
 47. The method of claim 23, wherein plasma etchingis used to form the spring.
 48. The method of claim 23, wherein wetetching is used to remove the first sacrificial layer and the secondsacrificial layer.
 49. The method of claim 23, wherein the gateinsulating layer is composed of an insulating material.
 50. The methodof claim 49, wherein the insulating material is silicon dioxide.
 51. Themethod of claim 23, wherein the gate insulating layer is thermallygrown.
 52. A method for making a contactless acceleration switch,comprising in combination: implanting a source, a drain, and a thresholdadjustment channel in a silicon substrate layer using ion implantation,wherein the threshold adjustment channel is located substantiallybetween the source and the drain, and wherein the threshold adjustmentchannel is doped to a level to cause the threshold adjustment channel toinvert when a mass moves substantially towards the silicon substratelayer; forming at least two silicon dioxide insulator posts on thesilicon substrate layer, wherein the at least two silicon dioxideinsulator posts are thermally grown, wherein wet etching is used to formthe at least two silicon dioxide insulator posts, and wherein thesource, the drain, and the threshold adjustment channel are locatedsubstantially between the at least two silicon dioxide insulator posts;forming a first sacrificial layer composed of silicon dioxide on thesilicon substrate layer substantially between the at least two silicondioxide insulator posts, wherein the first sacrificial layer isthermally grown, and wherein wet etching is used to form the firstsacrificial layer; forming the mass composed of doped silicon on thefirst sacrificial layer, wherein the mass is deposited using lowpressure chemical vapor deposition, and wherein plasma etching is usedto form the mass; forming a second sacrificial layer composed of silicondioxide shaped to provide a pattern for forming a spring, wherein lowtemperature oxidation is used to deposit the second sacrificial layer,and wherein wet etching is used to form the second sacrificial layer;forming the spring composed of doped silicon, wherein low pressurechemical vapor deposition is used to deposit the spring, and whereinplasma etching is used to form the spring; removing the firstsacrificial layer and the second sacrificial layer using wet etching,wherein the spring holds the mass substantially above the siliconsubstrate layer; and forming a gate insulating layer composed of silicondioxide, and wherein the gate insulating layer is thermally grown.